Homesteading the planets

Homesteading the Planets with Local Materials

April 28, 1998: Building lunar and Martian bases from local or
in situ (on-site) materials is a concept almost as old as space exploration.
Over the years, though, scientists and engineers have refined their ideas
of how to use materials and to work around the challenges of building a
factory with a long supply line.

Local materials

While the probable discovery of water on the lunar surface has stirred
great excitement, lunar soil offers a broad range of materials for building
- and for breathing, says Dr. David Noever of NASA/Marshall.

An attractive material for construction on the Moon is aerogel,
an exceptional insulator of heat and sound. Aerogel has been described as
frozen smoke since it weighs only slightly more than air. It's like foam
rubber, only the cells are incredibly tiny, and their walls are equally
thin. Sodium silicate (NaSiO2),
a common mineral on both the Moon and Mars, could be used to produce high-density
xerogels, or low-density aerogels.

Right: Dr. Dennis Tucker of NASA/Marshall sifts through
a sample of simulated lunar soil. Click for high-resolution
copies of this image.

Because they incorporate huge quantities of small pores, xerogels and
aerogels greatly retard the transmission of heat and sound. Each pore wall
acts as a new barrier. Silica-based aerogels also resist breakage, solar
ultraviolet light, and are also easily molded into shapes to fill gaps between
pressure walls and outer shells, or to serve as insulating pads.

Martian soil has most of the elements found in lunar soil (although in
different quantities), plus one not found in quantity on the Moon: carbon.
Large masses are readily available at the Martian poles in sheets of dry
ice that could be mined, chemically cracked using a process developed for
recycling crew air. This would yield oxygen for Mar's colonists to breath
and carbon for strong, lightweight carbon composite structures.

On a more conventional note, the most abundant element on the lunar surface
is oxygen - 42 percent of the total. It's not breathable but is locked up
in oxides of silicon (21%), iron (13%), calcium (8%), aluminum (7%), magnesium
(6%), and other materials (the actual composition varies with the region).
The metals are useful for buildings and even new spacecraft - the silicon
can be made into solar cells and windows, and the oxygen freed from the
soil can be breathed or used in rocket propellant.

But it's not as simple as scooping up dirt and applying enough heat to
break it down. Noever and his co-authors say that novel production methods
must be developed to extract metals from minerals on the Moon. Aluminum
processing is a conventional technology on Earth, but what works here won't
necessarily work up there. The standard methods require a constant supply
of electrodes and electrolytes - including sulfuric acid - and are difficult
to automate. A promising, simpler method tends to release toxic, corrosive
chlorine gas.

Titanium, another valuable aerospace metal, also is challenging, Noever
notes. If its oxide, rutile, can be extracted from ilemnite, one of the
common soils, then the rutile must be reacted with chlorine or fluorine
to produce pure titanium and oxygen.

Making glass is another attractive option, but it requires acids to leach
out unwanted metals, and high temperatures from a solar furnace to melt
the materials at 1,700 deg. C (3,092 deg. F) and produce silica glass.

(Co-authors with Noever are David Smith, Raymond Cronise, and Sandor
Lehoczky, all of NASA/Marshall, Laurent Sibille of the Universities Space
Research Association, and Scott Brown of the Southern Research Institute.)
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the report (text only).

Processing in low-g

One of the lessons from years of materials experiments in the weightlessness
of Earth orbit is that materials usually don't behave the way they do on
the ground. While we have lot of experience with materials at very low gravity
- 1/10,000th Earth gravity or less - we have little at intermediate gravity
- 0.125 on the Moon, or 0.369 on Mars.

"It is clear that significant changes are expected in the behavior
of liquid metals during processing," says Dr. Doru Stefanescu of the
University of Alabama in Tuscaloosa. "This, in turn, will affect the
price." Stefanescu and co-authors Dr. Peter Curreri of NASA/Marshall
and R.N. Grugel of the Universities Space Research Association have studied
how metals behave as they are processed
in the low-g conditions aboard the Space Shuttle.

A lack of an atmosphere will make it the melting of titanium and magnesium
cost-competitive with aluminum (for that environment).

Casting will be another matter. The lower surface gravity of the Moon
and Mars will make molten metals flow slower than they do on Earth. How
this may affect the inclusion of defects in the casting is unknown for now,
Stefanescu writes.

A house of glass

Once you figure that out, though, one of the products you might want
to make is glass fiber from which to weave a house. Fiberglass could be
used to reinforce composite structures, make cables, or form insulation
blankets (Beta cloth blankets on the Space Shuttle are made of woven glass
fiber).

Dr. Dennis Tucker and Edwin Ethridge, both of NASA/Marshall, have turned
simulated lunar soil into glass and then pulled it into hair-thin fibers
aboard NASA's low-g aircraft.

Right, Tucker examines the fiber-pulling apparatus during
a ground check. Click for high-resolution
copies of this image.

Lunar soil is too scarce to be used in large-scale experiments, so Tucker
and Ethridge used Minnesota Lunar Stimulants, terrestrial soils mixed to
match the compositions of different lunar soils. The apparatus they used
was relatively simple, a platinum furnace that heated the soil to 1,500
deg. C (more than 2,700 deg. F), and a take-up reel to pull the fiber through
a hole in the bottom of the furnace.

NASA's Vomit Comet is best known for flying roller-coaster trajectories
that leave everyone floating weightless for 20 seconds or so. It can also
be flown so everyone experiences lunar gravity.

With the furnace aboard the KC-135, Tucker - who is also working on next-generation fiber optics (ZBLAN) for advanced
communications - was able to pull glass fibers from molten lunar soil, but
with some problems. Gravity plays a role in the crystallization of the fibers
- glass is best when it has no crystals - and the final diameter of the
fiber. Pulling the fibers to just 10 to 15 microns wide (about half of what
was attempted in these tests) and coating them to protect the glass surface
should yield better results, Tucker and Ethridge say.

Solar cells

Of course, more of these grand activities can be attempted if you don't
have the power. While many of the industrial processes can use direct sunlight,
everything will require electricity. An attractive option is to make solar
cells from the abundance of silicon dioxide on the lunar surface or even
making up certain classes of asteroids.

Right: Solar cells power
a moon base and the surface mining operations. (Artist's concept by Pat
Rawlings of SAIC for NASA/Lewis)

Curreri and David R. Criswell of LPI write that thin film solar cells
could be made using lunar silica applied to substrates of metals derived
from other lunar manufacturing processes.

Shields up

Radiation has long been a concern for space travelers. Most crews have
been in low Earth orbit and protected by the magnetosphere; the Apollo crews
that went to the Moon were outside the magnetosphere for only a few days
and, luckily, during low solar activity.

Crews staying on the Moon or venturing to Mars will be exposed for weeks
or months at a time. Drs. Thomas Parnell and John Watts of NASA/Marshall,
and Dr. Tony Armstrong of SAIC in Prospect, Tennessee, took another look
at the problem to help define what needs to be done.

Cosmic rays come from the sun (solar energetic particles or SEP) and
the galaxy (galactic cosmic rays, or GCRs). They pose a hazard not just
to humans, but to the advanced electronics on which all modern space systems
depend.

Oddly, while the sun is a source of intense cosmic rays, it can also
protect space crews. The solar magnetic field can deflect cosmic rays, although
not as efficiently as the Earth's magnetic field does for low-Earth orbit
spacecraft

NASA is improving its models of cosmic ray fluxes, Parnell said, and
solar activity is the main variable.

As with bulletproof vests, the best way to stop cosmic rays is not with
the heaviest stuff around, but with lightweight materials. Heavy atoms are
shattered by cosmic rays and generate even more dangerous showers of secondary
radiation. The best shielding is lightweight materials that absorb the particles.
Heavy shielding can generate showers of secondary particles and radiation
that are more dangerous than the original. The ideal shielding would be
hydrogen, but in practice the best shielding may be water, food, and crew
waste since these are made of lightweight atoms like hydrogen, carbon, oxygen.